Guessing keys should never be an option, otherwise Bitcoin needs to upgrade its cryptographic functions. The supply will be shrinking in the sense of the asset/currency becoming deflationary at some point, as soon as 'lost Bitcoins > mining reward'. The limit of 21 million is arbitrary. Actually the current maximum in terms of units is 2,100 trillion, as you can divide every Bitcoin in 10^8 units. As a hard fork can update this denomination, there is nothing that limits Bitcoin to be adopted by a large audience.